The present invention relates to systems and methods for delivering liquid reagents to processing equipment. More particularly, it relates to the delivery of liquid reagents which are vaporized under controlled conditions as they enter a low pressure environment.
Chemical vapor deposition (CVD) processes are useful for forming thin, solid films on a heated substrate as a result of thermochemical vapor-phase reactions. Such processes are widely used in the semiconductor industry to form epitaxial and non-epitaxial layers on semiconductor substrates, commonly referred to as wafers. Typically, a vapor-phase reactant is introduced to a low pressure reactor, where the reactant reacts with the heated surface of the semiconductor wafer. Depending on the nature of the vapor-phase reactant, a variety of conductive, semiconductive, and dielectric layers may be formed.
The use of liquid reagents, while highly desirable for reasons of safety and environmental protection, presents a number of difficulties in the design and control of systems for reagent delivery. First, the volumes of liquid reagents being manipulated are very small compared to the corresponding gas volumes for the same amount of reagent. The ability to precisely control such small liquid volumes is inherently less reliable. Indeed, the liquid-phase volumetric flow rates being handled are typically in the range from about 1 ml/min. to 50 ml/min., which is below the capability of many flow valves and feedback flow controllers. A second problem arises from the presence of dissolved gases in the liquid reagents. Some dissolved gas is present in liquid reagents from most sources, and the use of pressurized nitrogen delivery systems (which is common in many liquid delivery systems) exacerbates the problem. In particular, the dissolved gases evolve from the liquid as the liquid is flashed across the metering valve into a lower pressure environment. Such evolving gases cause erratic two phase flow in the orifice of the metering valve, which in turn can cause erratic pressure variations upstream of the valve. These pressure fluctuations cause substantial flow variations through the valve which cannot be adequately offset by the control systems utilized. Such flow variations make it difficult to control the total amount of reagent delivered as well as to control the precise flow rate. Control of both these parameters is critical in most semiconductor fabrication processes.
To overcome these problems, many systems now utilize a vaporizer upstream of the flow control valve. In this way, the problems inherent in controlling very low volumetric flow rate of the liquid reagent may be avoided. While workable, such upstream vaporization requires very accurate temperature control of the vaporizer, vapor delivery lines, vapor flow rate measurement system, and metering valves, since the mass flow rate of the gas is a function of temperature and vapor pressure of the liquid. The flow controllers used in these systems require very accurate calibration for each type of reagent and cannot be used with another reagent without extensive recalibration. Thus, systems which use an upstream vaporizer are generally costly, complex, and of marginal accuracy.
Another approach for controlling the flow of a liquid reagent into a reactor relies on bubbling a known quantity of a non-reactive gas, usually nitrogen, through the liquid under controlled conditions of temperature and pressure. In this way, the effect of gas evolution as the liquid flashes across the metering valve can be more accurately taken into account and controlled. Such an approach, however, requires very accurate (and therefore costly) pressure and temperature control within the bubbler and can cause droplet entrainment as the gas enters the reactor.
For these reasons, it would be desirable to provide systems and methods which provide accurate metering and delivery of liquid phase reagents to semiconductor and other processing equipment, particularly low pressure reactors, such as chemical vapor deposition reactors. It would be particularly desirable if such systems could delivery a precisely measured volume of liquid reagent to the reactor without the necessity of upstream temperature control and that such systems could be used on a wide variety of reagents without the necessity of recalibration. Such systems should also be of simple design, be relatively inexpensive, and be capable of handling very low flow rates without substantial fluctuation, even when the liquids being delivered have relatively high levels of dissolved gases.